A hydraulic drive is proposed in, for example, DE 3,438,600 A1, wherein a hydraulic motor is provided which comprises a movable drive element fixedly connected to the tool, with the drive element being driven by alternative pressurization and pressure relief of drive pressure chambers along the lines of the tool movements. A follow-up control valve operating with electromechnically controllable reference position value setting for the movable drive element as well as with mechanical actual position value return indications thereof are also provided. For setting the reference position and value and return indication of actual position value, a threaded spindle is provided and a spindle nut, fashioned as a hollow shaft and meshing with the thread of the spindle are provided. As noted above, one of the spindle or nut of the spindle-nut system is driven by an electric motor so as to set to the reference position value and the other of the spindle or nut is driven along the lines of the return indication of the actual position in such a manner that, in case of equilibrium of the rate of change of the reference position value setting and the actual position value return indication, the spindle and the nut revolve at the same rotational speed and do not execute any axial relative movement with respect to each other. Within the scope of the follow-up control valve, a valve operating member is provided which partakes, when the spindle and the nut rotate at a different rotational speed, in relative movements of the nut with respect to the spindle and thereby drives the valve along the lines of increasing the throughflow cross-sections of supply paths through which the pressure medium flows for obtaining the respectively desired direction of movement, if the rate of change of the actual position is less than the change of the reference position value setting, and maintains these cross-sections constant if, and as long as, these rates of change are the same. The signals required for the motion control of the electric motor are produced by an electronic control unit with an interface for NC or CNC control.
In order to be able to determine whether and/or when the tool, driven by the drive mechanism has reached a specific position, either for determining whether the tool has executed its working stroke at all or for being able to accurately recognize the end point of its working stroke, for example, when the result of the operation is dependent upon actually reaching a definite end position, as in case of a press or embossing machine, a monitoring device is provided wherein the distance of the control valve operating member from its basic position is monitored and, once this distance falls below a predeterminable defined minimum value equivalent to the situation where the tool approaches dead center, a position-characteristic monitoring signal of a proximity switch is triggered. Thereby, a determination of the programmed dead center position of the tool is obtained which is sufficiently exact for many usage applications, with this determination indicating that the tool has executed its working stroke. It is merely necessary to effect suitable programming of the reference value setting device for adjusting the device to a specific stroke length of the tool and a specific material thickness of the workpiece and/or shaping depth thereof. The detection of the dead center position takes place with the aid of a proximity switch which responds when a rotary vane, driven via a transmission gear by the valve operating member, enters into a defined, preset position corresponding to the central position of the valve operating member assumed by the latter once equality has been reached of the reference position and the actual position. This is equivalent to reaching the dead center position of the tool.
In the conventional device, there is also a determination when the follow-up control valve is fully driven, that is, when the lag error .DELTA.S of the control circuit has become very large. This can be considered as evidence that a collision has occurred between the drive element and an obstacle, and, in such case, the drive mechanism is turned off. In this arrangement, the direction of movement of the drive element is also taken into account. In other words, it can be recognized by signal combinations generated by the monitoring device whether a collision has taken place in the forward or rearward direction.
Although it is definitely possible in the conventional drive mechanism to effect adaptation of its monitoring device to various usage purposes in that this device can be adjusted so as to vary threshold values of the lag error .DELTA.S, it is not possible to detect the lag error with respect to its amount as long as this error lies below the aforementioned threshold value. Consequently, there is a drawback insofar as a change in the lag error, for example, along the lines of an increase of the same, occurring during the operation of the machine equipped with the drive mechanism, cannot be detected.